In imaging workflows like cinema production workflow, broadcasting and digital imaging, color correction is an important step. For example, color correction is applied to raw film material after production to compensate for unwanted illumination color. Color correction is applied at the final phase of cinematographic post-production in order to realize specific moods according to the artistic intent of the director of photography. Color correction is also applied to photographs, scanned paintings or graphics before printing on paper. Color correction can be applied to images of a video content, to a single image of this video content, to still images or even for parts of such images, e.g. an object in an image.
Color corrections may include for example an increase of saturation, a change of color hue, a decrease of red tones or an increase of contrast. Color correction can be global to the whole of an image, to a set of images, to a specific region in one single image or even to all image regions in several images of a video content corresponding to a specific semantic unit.
During the process of color correction of images, the color operator has generally to keep in mind what will be the impact of the applied color correction on the target display device that will be used for the final reproduction of images. The following examples illustrate this problem. In case of digitalization of painting arts, a painting is photographed by a multispectral camera and then color corrected using a personal computer (PC). The operator verifies the applied color correction on the monitor of the PC while the final reproduction will be printing on paper. In this case, the PC monitor is the mastering display device used for the production of images and the paper printer is the target display device used to the final reproduction of images. Another example starts with the capture of images on argentic film. The film is then scanned, and then color corrected using a dedicated high-resolution color correction device. The color operator verifies the applied color correction on a high definition control monitor while the final color reproduction will be performed using a projector run on a printed film. Here, the control monitor is the mastering display device and the film printer and film projector are the final reproduction device. In another case, broadcast content in mastered on a high grade mastering monitor to be reproduced on the screen of consumer TV set.
Color characteristics differences between the mastering display device used for the production of images and the target display device used for final reproduction of these images should also be taken into account during color correction. Such differences may have an impact for instance on hue, saturation, contrast, intensity, dynamic range, and/or color gamut.
A solution to compensate for such differences of color characteristics is based on color management (CMM). Generally, to implement CMM, the color characteristics of the mastering display device and of the final target display device are measured, mathematically modelled and then, the differences of color characteristics are compensated using color transformations based on these mathematical models. CMM takes notably into account the color gamut of the devices. The color gamut describes the totality of reproducible colors of a display device. When an image to transform contains colors outside of the color gamut of a display device or close to the border of this gamut, the color transformation is adapted to map these colors inside this gamut, including the border of this gamut. Such a color transformation is then called color gamut mapping.
In general, color gamut mapping maps colors from a source color gamut into a target color gamut. The source color gamut can be content-dependent or device-dependent. When it is content-dependent, it generally means that all colors of the image(s) to map defines itself a source color gamut. When the source color gamut is device dependent, it is linked to device. Such a device can be a capture device such as camera or scanner. Such a device can be a mastering display device used to produce the image to map. The source color gamut can also be predefined, for instance according to a standard such as ITU-R BT.709. The target color gamut can be linked to a target display device used for the reproduction of images, such as a paper or a film printer, a projector or a TV set as already mentioned above. Such device comprises for instance LCD, OLED displays as those integrated in cell phones, tablets or TV sets. The target color gamut can also be predefined in a standard, as for example a standard used for transmission, compression and/or storage of images, for example ITU-R BT.709.
Generally, colorimetric color reproduction and non-colorimetric color reproduction are distinguished. Colorimetric color reproduction aims to reproduce colors on a target display device such that their colorimetry is identical or as close as possible to the colorimetry of these colors as displayed on a reference display device. The colorimetry of a color is measured in a colorimetric device-independent color space, as, for instance CIE XYZ color space. On the opposite, gamut mapping, by principle, involves non-colorimetric color reproduction since some of the colors are mapped, i.e. transformed. Non-colorimetric color reproduction can be guided by a series of criteria, compensating for the change of colorimetry, as, for instance, the preservation of color appearance, of contrast and/or of saturation.
Colors can be mapped along mapping trajectories anchored on anchor points defined in a given color space. As a compromise for minimization of loss in contrast and loss in saturation, colors can be mapped along straight lines as mapping trajectories. See for instance Montag and Fairchild in their paper entitled “Gamut mapping: Evaluation of chroma clipping techniques for three destination gamuts” published in 1998 at the IS&T/SID Sixth Color Imaging Conference in Scottsdale. The way how colors are mapped along such straight lines correspond for instance to a linear compression, a clipping, or, more complex, to non-linear functions as S-shaped functions.